Nitrogen oxides, primarily NO and NO.sub.2 are one of the major classes of air pollutants which are created during combustion processes. It is known that a two-stage, rich-lean combustion process will reduce NO.sub.x pollutants when fuels containing bound or fuel nitrogen (NO.sub.x precursors) are burned. In this process, the first stage is fuel-rich and in this stage, NO.sub.x pollutants normally formed from fuel nitrogen and atmospheric nitrogen are reduced to N.sub.2. Thereafter, the remainder of the air needed for completion of the combustion of unburned and partially burned fuel is added and the combustion is completed. The fuel-rich equivalence ratio (the ratio of actual fuel to actual air over the ratio of fuel-to-air necessary for stoichiometric combustion of .PHI.) is optimum between about 1.0-1.7 in order to obtain minimum NO.sub.x pollutants. The second volume of air is then added to the effluent from the fuel-rich stage to produce an overall equivalence ratio less than 1.0, usually about 3 to 15% excess oxygen. While such two-stage, rich-lean combustion substantially reduces the NO.sub.x pollutant emissions from the burning of solid fuels, the amounts of NO.sub.x pollutants are still comparatively high, particularly with solid fuels. It has also been suggested that further NO.sub.x reductions can be attained by operating a staged combustor with two fuel-rich stages followed by the fuel-lean stage, thus operating in a three-stage mode. While further reductions in NO.sub.x pollutant production are attained in this fashion, the NO.sub.x emissions are still comparatively high. Obviously, once an initial substantial reduction in NO.sub.x pollutants is attained by any form of NO.sub.x reduction, it is most difficult and in many cases, impossible, to obtain the last increments of reduction necessary to meet pollution control standards or provide a margin of safety between attainable results and pollution control standards.
Considerable work has also been done in an attempt to lower NO.sub.x pollutants by the addition of combustion catalysts, usually organo-metallic compounds, to the fuel during combustion. However the results of such attempts have been less successful than staged combustion.
Unfortunately, many fuels, particularly normally solid fuels, such as coal, lignite, etc., also contain substantial amounts of bound or fuel sulfur and the result is that conventional combustion produces substantial amounts of SO.sub.x pollutants which are also subject to pollution control. It has generally been the opinion of workers in the art that those conditions employed in staged combustion, particularly two-stage, rich-lean combustion, for NO.sub.x reduction will likewise lower the level of SO.sub.x emissions. However, it has been found in parallel work that little or no reduction in SO.sub.x emissions can be attained in a two-stage, rich-lean combustion process. As a matter of fact, it has been found that the presence of substantial amounts of sulfur in a fuel also has a detrimental effect on NO.sub.x reduction in a two-stage, rich-lean process.
A substantial amount of work has been carried out in the removal of sulfur from normally solid fuels, such as coal, lignite, etc. Such processes include wet scrubbing of stack gases from coal fired burners.
However, such systems are capital intensive and often unreliable. In addition, the disposal of wet sulfite sludge, which is produced as a result of such scrubbing techniques, is also a problem. Finally, the flue gases must be reheated after scrubbing in order to send them up the stack, thus reducing the efficiency of the system.
In accordance with other techniques, sulfur scavengers are utilized, usually in fluidized bed burners, to act as scavangers for the sulfur and convert the same to solid compounds which are removed with the ash. The usual scavengers in this type of operation include; limestone (calcium carbonate) and dolomite (magnesium-calcium carbonate) because of availability and cost. However, the burning techniques are complex and expensive to operate and control and the burner equipment is comparatively expensive.